1. Field of the Invention
The invention relates to a digital circuit arrangement for transforming an input digital picture signal onto a reference horizontal synchronizing signal raster derived from the system clock, the input digital picture signal being present at a system clock rate not locked with said input digital picture signal, said arrangement comprising a correction memory and an interpolator/decimator.
2. Description of the Related Art
If a digital picture signal is scanned at a system clock rate which is not locked with the picture signal or if this picture signal is already present at such a clock rate, this means that the digital picture signal, on the one hand, and the scanning raster, on the other hand, can be freely displaced with respect to each other. This particularly holds for phase shifts possibly occurring in the picture signal, for example, when the signal is displayed by a video recorder. It is often desirable to have the digital picture signal in an orthogonal raster, enabling it to be more easily processed, for example, in picture memories. To this end, the digital picture signal may be transformed onto a reference horizontal synchronizing signal raster which is derived from the system clock and is independent of the picture signal. Circuit arrangements which are capable of doing this are known from the prepublication "Fernseh und Kinotechnik" Vol. 40, no. 3/1986, pp. 105 to 111 and from prior German Patent Application 39 35 453 (PHD 89186), corresponding to U.S. patent application Ser. No. 07/599,336 , filed Oct. 17, 1990, now U.S. Pat. No. 5,121,207. In these two circuit arrangements, a correction memory and an interpolator/decimator are used for transforming the picture signal. In the arrangement according to the prior patent application they are controlled by means of a forward control.
Such a control particularly does not lead to a desired transformation of the digital picture signal when there is interference in this signal. Examples of interference are particularly, amplitude errors, noise superpositions or the flattening of the signal edges produced by limitations of the transmission bandwidth. Such interferences principally lead to a distortion of the correction value provided by the control and hence to a false transformation of the picture signal.